A 250 mL flask holds 0.484 g of a pure gas at 100.0 kPa and 25.0 °C…

Step 1. The amount. We want $n$, so $n = \dfrac{PV}{RT} = \dfrac{100.0 \times 0.250}{8.314 \times 298.15} = 0.01009\ \text{mol}$.

Step 2. The molar mass. $M = \dfrac{m}{n} = \dfrac{0.484\ \text{g}}{0.01009\ \text{mol}} = 48.0\ \text{g/mol}$.

Step 3. The density. $d = \dfrac{0.484\ \text{g}}{0.250\ \text{L}} = 1.94\ \text{g/L}$.